The present invention relates generally to boiler design, and in particular, to a new and useful Steam Assisted Gravity Drainage (“SAGD”) process boiler with natural circulation for operating with sub-ASME feedwater quality for oil sands, heavy oil and bitumen recovery.
The SAGD boiler design of the present invention has a basis in B&W drum boiler design, knowledge and standards. General boiler design standards are used and then expanded on where required to address specific design issues unique to SAGD.
Improvements have been made to enhance recovery of heavy oils and bitumens beyond conventional thermal techniques. One such technique, for example, is Steam Assisted Gravity Drainage or SAGD, taught by U.S. Pat. No. 4,344,485 issued Aug. 17, 1982 to Butler. This method uses pairs of horizontal wells, one vertically above the other, that are connected by a vertical fracture. A steam chamber rises above the upper well and oil warmed by conduction drains along the outside wall of the chamber to the lower production well.
The recovery of bitumen and subsequent processing into synthetic crude from the oil sands in northern Alberta, Canada continues to expand. Approximately 80% of known reserves are buried too deep to use conventional surface mining techniques. These deeper reserves are recovered using in-situ techniques such as Steam Assisted Gravity Drainage in which steam is injected via the horizontal wells into the oil sands deposit (injection well). This heats the bitumen, which flows by gravity to the other horizontal well lower in the deposit (production well) where the mixture of bitumen and water is taken to the surface. After the water is separated from the bitumen, it is returned to the process where, after treatment, it is returned to the boiler for re-injection into the well.
Re-use of the water resource is a key factor for both conservation and environmental regulations.
Even after treatment, however, the boiler feedwater can still contain volatile and non-volatile organic components as well as high levels of silica. Once Through Steam Generator (OTSG) boiler technology currently being used have experienced tube failures due to poor boiler feedwater quality. Further, the OTSG technology has exhibited limitations in steam quality produced and cost of operation such as high pumping power and cost of condensate handling to satisfy zero-liquid discharge requirements from SAGD plants.